4
4 Common Mode Damping is Isolated from DARM ETMXETMY Control Law u 2,2 u 2,3 u 2,4 u 1,2 u 1,3 u 1,4 0.5 The common mode damping injects the same sensor noise into both pendulums Since both pendulums are the same, this noise enters the test masses along only the common mode Thus, no damping noise to DARM from this loop! Warning: If one pendulum get’s bumped, the other will in part feel it too since both pendulums receive the same damping signal. Note that similar coupling already exists through DARM control. G1200774-v8 0.5 + Common mode damping

7
Comments up to this point The common and differential top TFs are not actually so surprising; the DOF that has the cavity control is the one that gets altered dynamics. So, if we rotate the quad longitudinal damping from local damping of each quad to global common and differential damping… Then, common mode damping noise is effectively eliminated since it is independent from the differential mode (where we measure GWs…). – Assumption: quads are identical. – Real life: noise is suppressed by how well the quads match. Ours are really close. See `Supporting Math’ slides. Residual differences might be tuned away by locally scaling the top mass damping for frequencies we care about. G1200774-v87

10
Simulated Damping Noise to DARM G1200774-v810 Red curve achieved by scaling top mass actuators so that TFs to cavity are identical at 10 Hz.

11
G1200774-v811 Local or global common damping to each stage Global differential damping damping to each stage

12
Where does the lonely differential long. mode come from? G1200774-v812 longitudinal ? Remaining top mass differential longitudinal mode. If we understand how the hierarchical control produces this mode, might we be able to design a hierarchical controller that also damps it? If so, then we can eliminate differential mode damping noise by turning this damping off. Since common mode damping couples weakly to DARM, virtually all longitudinal damping noise from these two quads would be gone from DARM.

13
Where does the lonely differential long. mode come from? Pendulum 1 f2f2 x4x4 The new top mass modes come from the zeros of the TF between the highest stage with large cavity UGF and the test mass. See more detailed discussion in the ‘Supporting Math’ section. This result can be generalized to the zeros in the cavity loop gain transfer functions (based on observations, no hard math yet). 13 G1200774-v8

16
Where does the lonely differential long. mode come from? 16 The top mass longitudinal differential mode resulting from the cavity loop gains on the previous slides. Damping is OFF! G1200774-v8

17
Where does the lonely differential long. mode come from? 17 The top mass longitudinal differential mode ringdown from the cavity loop gains on the previous slides. Damping is OFF! G1200774-v8

18
Conclusions Overall, OSEM sensor noise injection is minimized in 2 ways: – 1: some damping loops are removed entirely – 2: the remaining damping loops are applied to DOFs that couple weakly to DARM. 1) For the quads participating in DARM control (ETMs), you can design DARM to simultaneously damp the differential longitudinal modes. This removes the need for 1 out of 4 damping loops. 2) Quad common mode motion couples very weakly to DARM, so we can damp this separately from differential mode motion. If DARM control is extended to include all 4 quads, in principal we could isolate virtually ALL longitudinal damping noise from DARM. G1200774-v818

19
Conclusions cont. If DARM control cannot be extended to all 4 quads, we could still do common-differential mode damping between the ITMs. That would leave us with just 1 out of 4 longitudinal loops coupling to DARM, the differential mode ITM loop. Might design the damping of other DOFs and/or other cavities to include at least a subset of the 2 points above. E.g. Quad pitch damping, IMC length, etc. ESD not important to diff. damping ringdown for high UIM ugf. Noise, performance may not matter either…more analysis to be done on that point. G1200774-v819

20
Supporting Math 1.Dynamics of common and differential modes a.Rotating the pendulum state space equations from local to global coordinates b.Noise coupling from common damping to DARM c.Double pendulum example 2.Change in top mass modes from cavity control – simple two mass system example. G1200774-v820

23
Rotating all ETMX and ETMY local long. DOFs into global diff. and comm. DOFs G1200774-v823 Now, substitute in the feedback and transform to Laplace space: Grouping like terms: Determining the coupling of common mode damping to DARM

24
Rotating all ETMX and ETMY local long. DOFs into global diff. and comm. DOFs G1200774-v824 Solving c in terms of d and : Plugging c in to d equation: Defining intermediate variables to keep things tidy: Then d can be written as a function of :

25
Rotating all ETMX and ETMY local long. DOFs into global diff. and comm. DOFs G1200774-v825 Then the transfer function from common mode sensor noise to DARM is: As the plant differences go to zero, N goes to zero, and thus the coupling of common mode damping noise to DARM goes to zero.

32
Change in top mass modes from cavity control – simple two mass ex. 32 This is equivalent to m1m1 m1m1 m2m2 m2m2 k1k1 k2k2 x1x1 x2x2 C As we get to C >> k 2, then x 1 approaches this system m1m1 m1m1 k1k1 k2k2 x1x1 The x 1 to x 1 TF has one mode. The frequency of this mode happens to be the zero in the TF from f 2 to x 2. G1200774-v8

33
Change in top mass modes from cavity control – simple two mass ex. G1200774-v833 Discussion of why the single x 1 mode frequency coincides with the f 2 to x 2 TF zero: The f 2 to x 2 zero occurs at the frequency where the k 2 spring force exactly balances f 2. At this frequency any energy transferred from f 2 to x 2 gets sucked out by x 1 until x 2 comes to rest. Thus, there must be some x 1 resonance to absorb this energy until x 2 comes to rest. However, we do not see x 1 ‘blow up’ from an f 2 drive at this frequency because once x 2 is not moving, it is no longer transferring energy. Once we physically lock, or control, x 2 to decouple it from x 1, this resonance becomes visible with an x 1 drive. The zero in the TF from f 2 to x 2. It coincides with the x 1 to x 1 TF mode when x 2 is locked. m2m2 m2m2 f k2 k2k2 x2x2 f2f2 …

36
36 Cavity Control Influence on Damping ETMXETMY Control Law u 2,2 u 2,3 u 2,4 u 1,2 u 1,3 u 1,4 10 * Top to top mass transfer function * - Case 1: All cavity control on Pendulum 2 What you would expect – the quad is just hanging free. Note: both pendulums are identical in this simulation. longitudinal G1200774-v8

37
37 Cavity Control Influence on Damping ETMXETMY Control Law u 2,2 u 2,3 u 2,4 u 1,2 u 1,3 u 1,4 01 * Top to top mass transfer function * - Case 2: All cavity control on Pendulum 1 The top mass of pendulum 1 behaves like the UIM is clamped to gnd when its ugf is high. Since the cavity control influences modes, you can use the same effect to apply damping (more on this later) longitudinal G1200774-v8

38
38 Cavity Control Influence on Damping ETMXETMY Control Law u 2,2 u 2,3 u 2,4 u 1,2 u 1,3 u 1,4 0.5 * Top to top mass transfer function * - Case 3: Cavity control split evenly between both pendulums The top mass response is now an average of the previous two cases -> 5 resonances to damp. Control up to the PUM, rather than the UIM, would yield 6 resonances. aLIGO will likely behave like this. longitudinal G1200774-v8

41
Scratch: Rotating all ETMX and ETMY local long. DOFs into global diff. and comm. DOFs G1200774-v841 For DARM we measure the test masses with the global cavity readout, no local sensors are involved. The cavity readout must also have very low noise to measure GWs. So make the assumption that n x -n y =0 for cavity control and simplify to: Now, substitute in the feedback and transform to Laplace space: